KDE on a system with 128MB of RAM will be slower than molasses running uphill in the wintertime. Your system will absolutely crawl. Go with the Xfce desktop or a lightweight window manager. Vector Linux 5.9 Standard is your best choice.

If you can get some more RAM KDE would be a lot snappier that's for sure. Even if you get it to run without swapping on 128 MB, it will start swapping when you start for instance Firefox to do some browsing.

If you decide to try anyways, turn off as much unused services in VASM, and you might want to consider trimming and tweaking KDE a bit as well.